Wrought billets of metals and alloys for use in redeformation applications are conventionally produced by a hot working operation in which cast ingots are heated in a fuel fired or electrically heated box furnace and the heated workpiece is deformed, as for example by forging or rolling, to refine, consolidate and homogenize the grain structure of the metal and while changing the shape of the product to one more suitable for subsequent redeformation applications.
The grain size reduction or refinement which is achieved during such a hot working operation imparts improved strength and uniformity to the product and enables it to withstand additional deformation operations with less cracking. Additionally, the capability for examining the product for internal flaws by nondestructive testing means is considerably enhanced in a finer grained structure as compared to that in a coarser grained product.
When the heated workpiece is removed from the heating furnace for deformation, it immediately begins to radiate heat, resulting in a rapid reduction of the surface temperature of the workpiece. When deformation is begun, the contact of the cooler forging dies or rolls further chills the outer portions of the workpiece as the work progresses. As a result of this heat loss, it is necessary to periodically interrupt the hot working operation and to reheat the workpiece to a suitable working temperature so that the hot working operation can be continued.
Many metals and alloys are characterized by a relatively narrow range of temperatures within which they can be worked without initiating internal or external cracks and voids which detract from or destroy the usefulness of the product and increase product costs due to the expense of removing such defects and the net loss of usable products. For such metals and alloys, the relatively narrow working temperature range considerably limits the amount of hot working which can be carried out before it is necessary to stop for reheating of the workpiece, and typically it is necessary for the workpiece to be reheated a number of times in carrying out the hot working operation and reducing the workpiece to the desired size and shape.
In conventional hot working operations, the workpiece is returned to the gas fired or electrically heated furnace for reheating. During this step, as the workpiece is reheated back to working temperature, the grain structure of the product coarsens, thereby negating a portion or all of the grain structure refinement achieved during the previous deformation step. Thus, during a hot working operation the grain refinement of the material proceeds stepwise, and the net grain refinement which is achieved is the sum of the differences between the refinement achieved during each deformation step and the grain size increase during the subsequent reheating step.